Abstract
n an attempt to prepare new Fe catalysts for the dehydrocoupling of
amine‐boranes and to provide mechanistic insight, the paramagnetic FeII dimeric complex [Cp′FeI]2 (1) (Cp′=η5‐((1,2,4‐tBu)3C5H2)) was used as a precursor to a series of cyclopentadienyl FeII and FeIII mononuclear species. The complexes prepared were [Cp′Fe(η6‐Tol)][Cp′FeI2] (2) (Tol=C6H5Me), [Cp′Fe(η6‐Tol)][BArF4] (3) (BArF4=[B(C6H3(m‐CF3)2)4]−), [N(nBu)4][Cp′FeI2] (4), Cp′FeI2 (5), and [Cp′Fe(MeCN)3][BArF4] (6). The electronic structure of the [Cp′FeI2]− anion in 2 and 4
was investigated by SQUID magnetometry, EPR spectroscopy and ab initio
Complete Active Space Self Consistent Field‐Spin Orbit (CASSCF‐SO)
calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 1–6 were investigated as catalysts for the dehydrocoupling of Me2NH⋅BH3 (I) in THF at 20 °C to yield the cyclodiborazane product [Me2N‐BH2]2 (IV). Complexes 1–4 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra‐violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 1–4 proceeded via formation of the aminoborane Me2N=BH2 (II) as the major intermediate, whereas for 6 the linear diborazane Me2NH‐BH2‐NMe2‐BH3 (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me3N⋅BH3 were investigated in an attempt to identify Fe‐based intermediates in the catalytic reactions. The σ‐complex [Cp′Fe(MeCN)(κ2‐H2BH⋅NMe2H][BArF4] was proposed to initially form in dehydrocoupling reactions involving 6
based on ESI‐MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR
spectroscopic evidence. The latter also suggests that these complexes
function as precursors to iron hydrides which may be the true catalytic
species.
Original language | English |
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Pages (from-to) | 14127-14136 |
Journal | Chemistry - A European Journal |
Volume | 24 |
Issue number | 53 |
Early online date | 24 Aug 2018 |
DOIs | |
Publication status | Published - 21 Sept 2018 |
Keywords
- Amines
- Boranes
- Catalysis
- Dehydrocoupling
- Iron catalysts